The present invention relates to an inexpensive .gamma.'-precipitation strengthening Fe-base superalloy which is excellent in high-temperature strength and structural stability and used for heat resistant tools such as tools of hot extrusion presses and hot forging dies, engine valves, gas turbine engine parts, various kinds of coil or sheet springs, heat resistant bolts and so forth.
A .gamma.'-precipitation strengthening Fe-base superalloy known as A286 (JIS SUH660) (hereinafter referred to as A286) is used in a wide field as an inexpensive heat resistant alloy which can be used in a high-temperature range up to about 600.degree. C.
The composition of A286 is specified in JIS (Japanese Industrial Standard) as follows: up to 0.08% C (carbon), up to 1.0% Si, up to 2.0% Mn, up to 0.04% P, up to 0.03% S, 24.0 to 27.0% Ni, 13.5 to 16.0% Cr, 1.0 to 1.5% Mo, 0.10 to 0.50% V, up to 0.35% Al,1.90 to 2.35% Ti, 0.001 to 0.010% B (boron), and the balance of Fe.
On the other hand, improved alloys of A286 are proposed in JP-A-62-93353, JP-A-62-199752 and so forth. Further, an alloy of a broader composition range including A286 is proposed as an alloy for an exhaust engine valve in JP-A-56-20148.
However, for effective use of energy in consideration of the recent environmental problems, temperatures at which various kinds of heat resistant parts are used have been increased. For use in such a high temperature range, high-temperature strength of A286 is insufficient.
A286 is also used as various kinds of high-strength spring materials. For this use, however, when A286 is subjected to aging treatment after cold working, pseudo-stable .gamma.'-phase which contributes to strengthening is transformed into stable .eta.-phase, which results in a problem that a sufficient strength can not be obtained.
Moreover, any of the above-mentioned alloys proposed in JP-A-62-93353, JP-A-62-199752 and so forth as the improved alloys of A286 can not be said to have been sufficiently increased in strength as compared witch A286. Furthermore, although JP-A-56-20148 discloses the alloy including A286 for an exhaust engine valve which has a broader composition range, it is difficult to say that the alloy of JP '148 is considerably improved in strength as compared with A286 if Ni and Cr contents of the alloy are at about the same level as those of A286.